In a mobile communication, a signal level may instantaneously change in a receiving side due to a multi-path fading and the like, a received quality of an uplink signal by a base station and a received quality of a downlink signal by a mobile station may deteriorate drastically. There is a receive diversity and a transmit diversity, as a technique for reducing the deterioration of the received quality. The receive diversity is a technique to receive a signal by using a plurality of antennas in a receiving side. The transmit diversity is a technique to transmit a signal by using a plurality of antennas in a transmitting side.
Since the transmit diversity can reduce the change of the signal level, without increasing a circuit size and the number of antennas in the receiving side, the transmit diversity is mainly applied to a signal transmission in a downlink from a base station to a mobile station. Two main types of the transmit diversity are an open loop transmit diversity and a closed loop transmit diversity (refer to the non-patent document “3GPP RAN TS25.214 V6.2.0”, June 2004).
The open loop transmit diversity is a method in which a base station transmits a signal without receiving an instruction from a mobile station. The closed loop transmit diversity is a method in which a base station transmits a signal in accordance with an instruction from a mobile station. To be more specific, in the closed loop transmit diversity, the mobile station transmits, to the base station via an uplink, control data for instructing an antenna weight by which the base station multiplies a baseband signal, in order to obtain a higher gain by a signal combining. The mobile station performs an antenna verification for judging an antenna weight used by the base station, since an error may happen in the control data during transmission in the uplink (refer to the non-patent document “3GPP 25.214 V5.8.0, Annex A”, April 2004).
Moreover, a transmitter apparatus which can apply both the open loop transmit diversity and the closed loop transmit diversity is proposed (refer to the patent document “Japanese published unexamined application No. 2001-44900”, for example). And, in the closed loop transmit diversity, a technique for preventing the number of bits of the control data from being increased during soft handover, and the like (refer to the patent document “Japanese published unexamined application No. 2002-247629”, for example).
However, the conventional mobile communication system has used either the open loop transmit diversity or the closed loop transmit diversity statically, regardless whether or not the mobile station is during soft handover. But the number of radio links to be established between the mobile station and the base station becomes larger, while the mobile station is performing soft handover. In the inter-base-station soft handover, the mobile station established a plurality of radio links with a plurality of base stations. In the intra-base-station soft handover, the mobile station established a plurality of radio links with a base station.
Therefore, there is a problem that the received quality of the control data which is transmitted via an uplink and is received by the base station deteriorates, when the closed loop transmit diversity is used during soft handover. As a result, there is a case that the error of the control data is increased, and the base station cannot generate an optimal antenna weight. And, there is a case that the gain by the closed loop transmit diversity is decreased, because the number of the radio links is large.
Furthermore, since the transmission is performed by using a plurality of radio links, the received power per radio link which the mobile station receives is decreased. And, it becomes hard for the base station to estimate a BER (Bit Error Rate) of the control data, due to the deterioration of the received quality of the uplink. As a result, there is a case that an accuracy of the antenna verification which is performed by the mobile station deteriorates.
Because of the above deterioration factor, there is a possibility that the received quality deteriorates when the closed loop transmit diversity is applied during soft handover more than when the closed loop transmit diversity is not applied during soft handover. As a result, there is a possibility that a capacity of a downlink deteriorates. Such a problem is accentuated in the inter-base-station soft handover.